In recent years, activity directed towards the development and improvement of energy sources and energy storage devices has been rapidly expanding. For example, a considerable amount of work has been directed toward the development of new active electrode materials for battery cells and fuel cells. The use of certain transition-metal chalcogenides as the active material in the positive electrode of secondary cells has yielded a high capacity per unit weight and a favorable discharge characteristic. However, it is now desirable to see if further improvements in cell characteristics and fabrication techniques may be obtained by investigating other aspects of the cell including electrode binders. Binders are electrochemically inactive materials commonly added to the active electrode material to increase the structural integrity and porosity of the electrode.
Maintenance of structural integrity is of considerable importance in the operation of an energy cell. It contributes to stability of operation (retards decrease in voltage and capacity with age) and insures greater lifetime of the battery. It is particularly desirable for secondary batteries which may be repeatedly cycled during operation.
The material used as a binder in the electrode should possess several properties. For example, it is desirable that it be chemically inert with respect to the materials present within the battery cell or fuel cell. The binder should not be susceptible to reduction or oxidation. To maximize energy capacity per unit weight or volume, it is also desirable to obtain good structural integrity with minimum amount of binder material. Additionally, it is advantageous to use a binder material that is light in weight and that forms an intimate and cohesive mixture with the active electrode material. It is also desirable that the electrode structure yield a large surface area per unit weight of active material so as to enhance cell reactions.
In terms of manufacturing electrodes for energy cells, it is desirable to avoid use of solvents and to avoid use of high temperatures and pressures. Solvents are often difficult to dispose of and invariably add to the cost of manufacture. They also add to environmental problems. Avoidance of high temperatures and pressures is desirable so as to insure that the active material is not adversely affected.
At the present time, there are a number of materials used as binders in the electrodes of cells and fuel cells. The most commonly used appear to be polyethylene and polytetrafluoroethelyne. Polytetrafluoroethelyne is frequently referred to as either PTFE or TEFLON. Other fluorocarbon materials have also been used as binders in electrodes. For example, U.S. Pat. No. 4,118,334 discloses a method for forming a cathode structure using a fluorocarbon polymer binder. The method described involves the use of a 50 percent isopropanol solution in water and requires that the electrode mixture be cured at high temperature for long periods of time. While perfectly adequate for some purposes, it is desirable in some cases to be able to form the electrode structure at moderate temperatures and without the use of solvents.